Add Python 3 Train unit tests

This is a mechanically generated patch to ensure unit testing is in place
for all of the Tested Runtimes for Train.

See the Train python3-updates goal document for details:
https://governance.openstack.org/tc/goals/train/python3-updates.html

Note that openstack-python35-jobs is retained since this charm is supported
on Xenial.

Change-Id: I80a54de2107c44694b5bd062988b67aae3eda1fe
Story: #2005924
Task: #34228

.zuul.yaml

@@ -2,3 +2,4 @@
     templates:
       - python35-charm-jobs
       - openstack-cover-jobs
+      - openstack-python3-train-jobs

tox.ini

@@ -3,7 +3,7 @@
 # within individual charm repos.
 [tox]
 skipsdist = True
-envlist = pep8,py3
+envlist = pep8,py37
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
@@ -37,6 +37,11 @@ basepython = python3.6
 deps = -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
+[testenv:py37]
+basepython = python3.7
+deps = -r{toxinidir}/test-requirements.txt
+commands = stestr run {posargs}
+
 [testenv:pep8]
 basepython = python3
 deps = -r{toxinidir}/test-requirements.txt